1. Field of the Invention
The present invention relates to semiconductors. More specifically, the present invention relates to semiconductor structures that include Aluminum Antimonide (AlSb) and lattice matched solid solution semiconductor materials so as to produce solar cells.
2. Description of Related Art
Solar cells or photovoltaics (PV) manufactured from semiconductor materials are based on absorbing photons of light so as to promote valence electrons of the semiconductor to the conduction band to enable such electrons to move freely through the semiconductor. At the same time, the holes left by the yielded electrons can jump from core to core, thus forming positive charge carriers which can also move easily through the valence band of the semiconductor material. Such a mechanism thus generates electron-hole pairs so as to produce a current that can be harvested to charge batteries, operate motors, and to power a wide variety of electrical loads.
Because of the concerns over limited resources, efforts have been ongoing to increase the output and/or the efficiency of PV cells. One such arrangement includes stacking materials to create multi-junctions (grouping a predetermined number, often greater than about 2, different p-n junction semiconductor materials) so that predetermined materials having different energy bandgaps can absorb a different part of the energy distribution from the sun. In such an arrangement, the top layers absorb higher-energy photons, while transmitted lower-energy photons are absorbed by the lower layers of the configured device. Background information for such devices is described and claimed in U.S. Pat. No. 6,891,869 B2, entitled “Wavelength-Selective Photonics Device,” issued May 10, 2005 to Augusto, including the following, “A device comprising a number of different wavelength-selective active-layers arranged in a vertical stack, having band-alignment and work-function engineered lateral contacts to said active-layers, consisting of a contact-insulator and a conductor-insulator. Photons of different energies are selectively absorbed in or emitted by the active-layers. Contact means are arranged separately on the lateral sides of each layer or set of layers having the same parameters for extracting charge carriers generated in the photon-absorbing layers and/or injecting charge carriers in the top photon-emitting layers. The device can be used for various applications: wavelength-selective multi-spectral solid-state displays, image-sensors, light-valves, light-emitters, etc. It can also be used for multiple-band gap solar-cells. The architecture of the device can be adapted to produce coherent light.”
In addition, solar cells, such as, GaAs (a=5.6533 Å) and Ge (a=5.6575 Å) stacked devices have been arranged in lattice matched configurations (lattice mismatch is on the order of 0.074%) so as to minimize surface dislocations, i.e., crystal defects at the interface of the stacked layers. The presence of such crystal defects reduces the minority-carrier lifetimes in the bulk of the layers, increases the surface recombination velocity at interfaces and creates possible shunting paths, all of which can reduce the efficiency of PV devices, and in general, degrade device performance. Further, multi-junction solar cells and other optoelectronic devices having these crystal defects degrade under radiation.
Background information for perfectly lattice-matched (PLM) semiconductor layers is described in U.S. Pat. No. 6,586,669 B2, entitled “Lattice-Matched Semiconductor Materials for Use in Electronic or Optoelectronic Devices,” issued Jul. 1, 2003 to King, et al, including: “In this context, PLM means that the lattice mismatch between the PLM cell and growth substrate is less than 0.074%. If specified, PLM may also refer to a difference in lattice mismatch between the PLM cell and an adjacent cell of less than 0.074%.” Such lattice mismatching is emphasized in a 2005 Solar Energy article by M. Yamaguchi et al, entitled, “Multi-junction III-V solar cells: current status and future potential,” by the following: “Although 0.08% lattice-mismatch between GaAs and Ge was thought to be negligibly small, misfit-dislocations were generated in thick GaAs layers and deteriorated cell performance.”
However, in the context of solar cell performance, it is reported by Burnett in a 2002 document entitled, ‘The Basic Physics and Design of Multijunction Solar Cells” that “work at NREL showed that lattice mismatching as low as ±0.01% causes significant degradation of photovoltaic quality.” It is, therefore, very important in multi-junction solar cell operation to use semiconductor compositions that are latticed matched.
Such multi-junction lattice matched cell layers can be stacked mechanically or the layers can be grown monolithically, typically by metal-organic vapor phase epitaxy (MOVPE) or molecular beam epitaxy (MBE). Background information on similar lattice matched devices can be found in U.S. Pat. No. 6,300,558 B1, entitled “The present invention relates to a high efficiency solar cell that can be used as an energy source of an artificial satellite, etc. and, more particularly, a lattice matched solar cell using group III-V compound semiconductor, epitaxially grown on a germanium (Ge) substrate, and a method for manufacturing the same.”
Accordingly, a need exists for solar cell configurations that include controlled atmospherically annealed high purity AlSb single crystals so as to efficiently couple the sun's energy distribution. The present invention is directed to such a need.